In a current wireless communication system, a variety of methods aiming at high-quality and high-capacity data transmission by employing limited frequency resources have emerged. Further, for the purpose of this high-peed multimedia data transmission, there is an increasing need for an effective countermeasure with respect to a fading phenomenon occurring in a wireless channel.
In recent years, a variety of researches have been done into Multiple Input Multiple Output (MIMO) technique employing multiple antennas, which will be applied to the next-generation mobile communication system for ultra-high speed multimedia data transmission, and Orthogonal Frequency Division Multiplexing (OFDM) technique that can efficiently cope with a frequency selectivity of a channel.
Spatial Division Multiplexing (SDM) scheme is an example of the MIMO technique. A transmitter transmits different data through respective transmit antennas, and a receiver identifies the data through proper signal processing such as interference removal and diversity technique. As the number of the transmit antennas and the number of the receive antennas are increased, the channel capacity increases linearly. Thus, multiple antenna technique is an indispensable research subject for a current wireless communication system requiring a high transmission rate.
In order to effectively transmit high-speed data, inter-symbol interference occurring at the time of high-speed transmission or frequency selective fading occurring due to multiple path interference must be overcome. The OFDM scheme can be used to effectively remove the frequency selective property of a channel. Furthermore, spectral efficiency can be increased by the use of multiple carriers having a mutual orthogonality, and a modulation process at a transmitter and a demodulation process at a receiver can be implemented at high speed by using Inverse Fast Fourier Transform (IFFT) and Fast Fourier Transform (FFT).
A closed-loop system for providing a feedback channel from a receiver to a transmitter has been developed to improve the performance of the system. When Channel State Information (hereinafter, referred to as ‘CSI’) is fed back to the transmitter, the transmitter can control several system parameters using this information, resulting in a maximized performance.
However, transmitting the CSI for the entire channels can be increase signaling overhead and user mobility can degrade accuracy of the CSI. Therefore, there is a need for a criterion for determining a Modulation and Coding Scheme (hereinafter, referred to as ‘MCS’) under high user mobility.
An Adaptive Modulation and Coding (hereinafter, referred to as ‘AMC’) scheme is a technique for increasing the link performance by controlling a transmission power level, a modulation level and/or a code rate in a transmitter using current CSI. When the channel state is good, the data transmission rate is increased, and when the channel state is not good, the transmission rate is decreased in order to support efficient transmission. Consequently, a mean transmission rate can be increased.
The AMC scheme can be used to optimize the data transmission on the basis of accurate CSI. If CSI used in a transmitter does not exactly match that used in a receiver, the AMC scheme experiences severe performance degradation. If channel error is very great, the performance of the AMC scheme can be lower than that of an open-loop transmission system. In a wireless communication system, user mobility is one of main reason for the channel error. To provide high-capacity and high-quality services under high user mobility, a method of minimizing performance degradation of the AMC scheme is needed.
In a cellular system, many users exist within one cell. A performance gain can be obtained by supplying information to a number of users having different channel gain values at an appropriate time. This is called a multi-user diversity gain. Furthermore, the performance of the system can be increased by applying the AMC scheme on a user basis. Accordingly, there is a need for a method of determining MCS for multiple users and efficiently scheduling the respective users based on the MCS, thus increasing a multi-user diversity gain.